(1) Field of the Invention
The present invention relates to a multitone thermal transfer recording method and apparatus for recording an multitone image represented by digital tone data by electrifying a heating element for a period in response to the level of tones in the data, and particularly to an improvement for reducing a period required for transferring the digital tone data to a shift register of a recording head and an improvement in the temperature compensating technique.
(2) Description of the Prior Art
When printing a multitone image, it is necessary to transfer digital tone data representing the level of tones of the image to the recording head. In a commonly practiced transfer method, the data are transferred the number of times corresponding to the number of tone levels by cycles of a unit electrification period as disclosed in Japansese Laid-Open patent application (kokai) 57-48868.
According to this transfer method, however, the data transfer takes a long time, and that in proportion to the number of tones. The long data transfer period is contrary to high speed printing since the thermal transfer recording apparatus will then be operable at a printing speed governed by the long transfer period.
A proposal has been made in Japansese Laid-Open patent application 57-57682 to reduce the data transfer period. The method disclosed in this publication is based on a basic principle of transferring digital tone data digit by digit, whereby the times of data transfer are reduced and the data transfer period is shortened. In this method, however, the heating element is not electrified continuously though electrified for periods each corresponding to the number of tones. The heating element radiates heat during breaks in the electrification, and this results in a density reversal phenomenon among part of the data so that data having a large number of tones is printed with a less density than data having a small number of tones.
Further, temperature compensation is provided for conventional thermal printers in order to record image data with good tone reproduction. Such technique is at present known from Japansese Laid-Open patent applications 59-38078 and 57-205179.
According to the disclosure in Publication 59-38078, the recording head receives a strobe signal whose pulsewidth is variable with temperatures of the recording head. This technique has the problem that the data transfer period cannot be secured when the pulsewidth of the strobe signal becomes very small.
On the other hand, 57-205179 applies a chopped pulse signal to the heating element, and its pulse duty ratio is varied with head temperatures. This method realizes a constant strobe time, and therefore presents no trouble to the data transfer unlike the known method noted above. However, this technique gives no consideration to the chopping frequency. That is to say this technique is effective for compensating temperatures in printing black and white binary data, but leaves room for improvement to be fit for practical use as temperature compensating means where data having a large number of tones such as 32 tones, 64 tones and so on are printed.